The present invention relates to methods of attenuating electromagnetic signals, and more particularly, to attenuation networks.
Step attenuators are well known in the electronics industry. A common step attenuator has a number of individual attenuators, or attenuator cells, selectively connected in series. Each attenuator cell attenuates an input signal by a predetermined value, which is typically measured in decibels (dB). A step attenuator may be designed to include n attenuator cells having attenuation values of 2k (dB), where k=0, 1, 2, . . . , n. An individual switch pair is provided for each attenuator cell. By opening and closing the switches, it is possible to select any given value of attenuation up to the sum of the attenuation of all of the attenuator cells.
FIG. 1 shows a known step attenuator 8 configured as described above. Five attenuator cells 11, 12, 13, 14, 15 are arranged in series between an input 10 and an output 19. Attenuator cells 11, 12, 13, 14, and 15 respectively have attenuation values of 1 dB, 2 dB, 4 dB, 8 dB, and 16 dB. Each attenuator cell employs a T- or xcfx80-connection of three resistors (not shown) which are selected to provide the desired attenuation and to equalize the input/output impedance of the attenuator cell with the input/output impedance of step attenuator 8. Attenuator cell 11 has two switches 16, 17 connected thereto. Switches 16, 17 are preferably single-pole double-throw type switches and direct an input signal either through attenuator cell 11 or through a bypass line 18, which bypasses the attenuator cell. Attenuator cells 12, 13, 14, 15 have similar switches and bypass lines such that control of the switches enables an operator to selectively connect any desired combination of switches, which in turn permits step attenuator 8 to attenuate a signal between 0 dB and 31 dB (in 1 dB steps) as desired.
One problem with step attenuator 8 is that significant losses are incurred in the switches and transmission lines (termed xe2x80x9cinsertion lossesxe2x80x9d) even when the input signal does not pass through the attenuation cells. To solve this problem, another type of step attenuator has been developed, shown in FIG. 2, that includes a bypass line 22. When attenuation is desired, switches 20, 21 are positioned as shown in FIG. 2 to direct the input signal through the attenuator cells. When no attenuation is desired, switches 20, 21 are positioned to direct the input signal through bypass line 22. This arrangement reduces the insertion losses because the non-attenuated input signal does not pass through all of switches 17. However, such an arrangement does not address step error, which is also present in known step attenuators. Step error occurs when the difference between two attenuation levels is too small to differentiate between the levels. For instance, if the step error between attenuation levels 1 dB apart (xe2x80x9c1 dB step errorxe2x80x9d) is more than +/xe2x88x920.5 dB, it will be difficult to tell which attenuation level is desired. To minimize 1 dB step error to less than +/xe2x88x920.35 dB, bypass line 22 should be insulated from the line having the attenuator cells to a level of at least 70 dB. This may be accomplished by including a multiple diode circuit in each of switches 20, 21. However, the multiple diode circuit requires more area and increases the insertion losses in the attenuator such that the insertion loss through the switches is greater than 1.0 dB. The first 1 dB step, therefore, is difficult to attain.
In practical high-power applications of known step attenuator designs, further errors are introduced due to parasitic capacitances between the high-power resistors making up the attenuators and the underlying ground plane. Additional errors are caused by parasitic reactive components of high-power PIN diodes included in the attenuator design.
It is possible to miniaturize the design of a step attenuator by fabricating the attenuator from a stripline or microstrip line having a small dielectric substrate thickness and a high relative dielectric constant. Such a design of the diode sections and attenuator cells produce additional parasitic capacitances between the print circuits (or pads) of these elements and the underlying ground plane. To compensate for the parasitic capacitances or reactances, conventional step attenuators require additional tuning and matching elements, transformers, and reactive stubs. These additional components cause the step attenuators to have a narrow frequency bandwidth, occupy a larger area, and require more precise manufacturing tolerances due to the resulting increased sensitivity.
It is therefore an object of the invention to provide a step attenuator with a step error reduced so that each desired attenuation level is differentiable from other attenuation levels.
It is another object of the invention to reduce parasitic capacitances within a step attenuator.
It is another object of the invention to provide a step attenuator with minimized insertion losses.
The invention provides a step attenuator for use in attenuating an electromagnetic signal. The step attenuator includes a first path having a plurality of attenuator structures provided therein, each attenuator structure being selectively actuated to permit the signal to pass therethrough. A second path is disposed in parallel with the first path, the second path permitting the signal to selectively bypass the first path. A third path is disposed in series with the first and second paths and includes at least one attenuator structure that is selectively actuated to permit the signal to pass therethrough.
Additionally, the invention provides a high-power RF step attenuator. The attenuator includes a first path that has a plurality of attenuator structures provided therein, each attenuator structure being selectively actuated to permit an RF signal to pass therethrough. The first path has a selectively actuable first path input switch disposed at one end of the first path to selectively permit the signal to enter the first path. A second path is disposed in parallel with the first path. The second path permits the signal to selectively bypass the first path. A third path disposed in series with the first and second paths and includes at least one attenuator structure that is selectively actuated to permit the signal to pass therethrough. At least one of the attenuator structures further includes: a first switch disposed on an input side of the attenuator structure, the first switch selectively permitting the signal to enter the attenuator structure; a second switch disposed on an output side of the attenuator structure, the second switch selectively permitting the signal to exit the attenuator structure; a bypass line that permits the signal to bypass the first and second switches; and a third switch disposed along the bypass line to selectively permit the signal to pass through the bypass line. In one embodiment, there are a total of n attenuator structures, wherein a first attenuator structure provides p dB of attenuation to the signal. Each of the second through nth attenuator structures provides a level of attenuation that is substantially double (in dB) that of the previous attenuator structure such that the nth attenuator structure provides a level of attenuation equal to 2nxe2x88x921p dB. The first attenuator structure and the nth attenuator structure are disposed within the third path.